Over the past two decades, new flat panel display technology based on light emission from thin layers of small organic molecules (organic light-emitting diodes or OLEDs) or conducting polymers (polymer light-emitting diodes or PLEDs) has emerged. As used herein, the term OLED will be used to refer to organic electroluminescent devices, including both small molecule OLEDs as well as PLEDs. Compared to liquid crystal-based displays (LCDs), this technology offers higher contrast displays with lower power consumption, and response times fast enough for video applications. Displays based on OLED technology exhibit a much wider viewing angle than liquid crystal displays (LCDs). OLED technology has the additional advantage over LCD technology in that OLEDs do not require back lighting whereas LCD technology does require back lighting. Currently, more than seventy companies worldwide are developing display technologies based on OLED structures. Sales of displays based on OLEDs, such as car radios, mobile phones, digital cameras, camcorders, personal digital assistants, navigation systems, games, and subnotebook personal computers, are forecast to grow to more than one billion dollars in 2005. See, for example, Society For Information Display, Short Course S-4, Fundamentals of OLED Displays, Jun. 3, 2001.
The basic device configuration for OLEDs is a multilayered or sandwich-type structure comprising a substrate, a transparent anode, two or more organic layers with different charge transport or luminescence characteristics, and a metal cathode. The morphology of the organic layers typically ranges from mesomorphous (e.g., semi-crystalline) to amorphous. Unlike inorganic LEDs, there are no lattice matching requirements with OLEDs, which greatly widens the types of substrates that can be used and the types of materials that can be combined together into devices. Use of multiple organic layers in the device geometry facilitates charge injection at the organic-electrode interface, leading to lower driving voltages. In addition, use of multiple organic layers allows the buildup of electrons and holes (and therefore, the location of the emission zone) to occur away from the electrodes, which significantly improves the efficiency of the device.
A typical organic light emitting device 30 in accordance with the prior art is shown in FIG. 1. Organic light emitting device 30 comprises a substrate 32. Substrate 32 can be made from a variety of materials, including but not limited to, glass, quartz, and plastic. Anode 34 overlays substrate 32. A typical material used to make anode 34 is indium tin oxide. A hole transport region 36 composed of a hole transport material (HTM) overlays anode 34, a mixed region 38 comprising a mixture of a hole transport material and an electron transport material overlays hole transport region 36, and an electron transport region 40 composed of an electron transport material (ETM) overlays mixed region 38. A cathode 42 overlays electron transport region 40 and a protective layer 44 overlays cathode 42.
Not all electroluminescent devices have precisely layers 36, 38, and 40. In fact, this is one of the advantages of OLEDs. For convenience, the term “organic layer” is used to refer collectively to layers 36, 38, and 40. Thus, in some structures, the “organic layer” includes discrete layers 36, 38, and 40 whereas in other devices the “organic layer” comprises any number of layers that, collectively, are equivalent to layers 36, 38, and 40 of FIG. 1. For instance, in some instances, five or more layers could, together, form the “organic layer.” In mixed layer 38 (FIG. 1), one of the hole transport material and the electron transport material is an emitter. Upon application of an electrical current, the organic electroluminescent device radiates light generated by recombination of electrons and holes in the organic materials used to make layers 36, 38, and 40.
An OLED is a current-driven device. That is, the intensity of the output light is directly proportional to the electrical current flow through the device. An OLED display, therefore, requires the control and modulation of electrical current levels through individual elements (pixels) in order to display text or graphic images. There are two general architectures for addressing pixels in an OLED: passive matrix and active matrix. Referring to FIG. 2, the passive-matrix OLED display is formed by dividing anode layer 34 into columns and cathode layer 42 into rows that intersect the anode columns. In typical implementations, the columns provide the data signal while the rows are addressed one at a time. The current flow through a selected row is typically pulsed to a level that is proportional to a level that is a function of the total number of rows in the display.
Although passive-matrix OLED displays are relatively simple to construct with intersecting anode columns and cathode rows, their fabrication requires patterning the reactive cathode layer 42 without affecting the properties of underlying organic layers (e.g., FIG. 1, layers 36–40). In one known approach, illustrated in FIG. 3, an integrated shadow mask is used to accomplish the task of dividing cathode layer 42 into rows. In the integrated shadow mask method, anode layer 34 and an integrated shadow mask are deposited on substrate 32 and patterned to form undercut pillars 310 using photolithography before the organic layers 36–40 and cathode 42 are deposited. In particular, a positive photoresist that, under certain circumstances, can switch to a negative tone is used to create undercut pillars 310. The effect of undercut pillars 310 is that they isolate the cathode layer 42 in each respective pixel region 302 from neighboring pixel regions. As is illustrated in FIG. 3, each pillar 310 is undercut to insure isolation of respective rows of cathode 42.
While passive-matrix OLED devices find many applications in consumer and industrial products, there is a drawback with such devices. They are not easily fabricated in a manner allowing for consistent device reliability and appropriate yield. Each pillar 310 has an undercut slope profile to avoid the need for angle evaporation and to improve cathode row isolation. The manufacture of such undercut photoresist structures is not reliable using known lithographic techniques. See, for example, U.S. Pat. No. 6,107,736 to Shi et al. In some instances the slope profile of the shadow mask used to make pillars 310 is achieved using a form of positive photoresist that is baked at specific temperatures in order to reverse the resist to a negative photoresist. Each batch of photoresist used in such processes needs to be calibrated in order to identify the temperature at which this conversion from positive resist to negative resist is achieved. Such calibration is time consuming and expensive. Furthermore, the integrated shadow mask method is sensitive to lines and features (e.g., patterned anode 34) because the resist is sensitive to anything that changes the aerial image in the resist. Additionally, the integrated shadow mask method involves several complex steps. In some cases, two exposures are required. The first exposure is a more penetrating exposure that is used to shape the bottom of the shadow mask (e.g., the bottom of pillar 310). Then, the second exposure is less penetrating and is used to shape the top of the shadow mask. In some instances, the shadow mask is patterned in a single exposure. Yet another drawback with the integrated shadow mask method is that the method results in an increase in the cross-talk associated with device leakage between the anode 34 and the cathode 42. The leakage is mainly caused by the poor surface coverage of organic electroluminescent medium at the edge between the shadow mask (310) and anode 34. See, for example, U.S. Pat. No. 6,107,736 to Shi et al.
One of the drawbacks in the manufacture of passive matrix OLEDs has been discussed. Another hurdle in the manufacture of OLEDs arises in the case where the OLED based display is a color display. In such instances, each pixel in the color display is represented by three different colors, which approximate the three primary colors, red, green, and blue. For proper operation of the color display, each color in each pixel needs to be isolated from the other colors.
A schematic of a typical active matrix OLED color display in accordance with the prior art is illustrated in FIG. 4. There are thin film transistors 402 embedded in a substrate 32. Anode layer 34 overlays substrate 32. Emissive layer 410 (the organic layer) in the OLED color display is separated into discrete color isolation wells. Each color isolation well is filled with a predetermined emissive polymer or small molecule dye. Representative polymers include, but are not limited to, poly(phenylene vinylene) derivatives (Cambridge Display Technology, Cambridge England) and poly(fluorene) derivatives (Dow Chemical, Midland, Mich.). A barrier 420 is used to shape the boundaries of each color isolation well. Typically, an isolation barrier 430 is overlayed on top of each barrier 420. Barrier 420 is typically made out of a material such as SiO2. Isolation barrier 430 is typically made out of photoresist such as a poly(methyl methacrylate) (PMMA) or polyimide. One representative polyimide that can be used to form isolation barrier 430 is Dupont Pyralin 2411 photosensitive polyimide. See, for example, White et al., 1995, Appl. Phys. Lett. 66, p. 2072; and Pottiger, Proceedings of the 38th Electronic Components Conference, presented at the IEEE Conference, Los Angeles, Calif. (IEEE, New York, 1988), p. 315. Finally, cathode 42 is overlayed on the device to complete the color based active matrix OLED.
A major hurdle in the fabrication of OLED color displays is the population of the color isolation wells with polymer or small molecule dye. Typically, such polymers or small molecule dyes are deposited using techniques such as ink jet printing. However, there are two setbacks associated with this process, containment and wetability. Containment issues involve the need to precisely deposit each dye in the correct color isolation well. Wetability issues relate to the need to match the properties of the color isolation well with the properties of the polymer or small molecule dye so that the dye coats the bottom of the well in a uniform manner. If the surface properties (e.g., hydrophobicity) of the color isolation well do not match the surface properties of the polymer or small molecule, then the dye will bead up within the color isolation well, resulting in unfavorable optical properties in the color display. If wetability is optimized, then the dye will more uniformly coat the color isolation well. Containment depends upon the ability to first deposit the right ink into the right color isolation well and second to trap the ink in the well.
Currently, wetability and containment issues are addressed using a positive slope 460 (FIG. 4) in isolation barrier 430 in order to trap polymer in the color isolation well. Further, various surface treatments are used to control the surface properties of structures 430 to assure adequate wetting of the surface by the light emitting polymer. However, as mentioned above, isolation barrier 430 is made out of patterned photoresist such as PPI or PMMA. Such photoresists are known to have unfavorable wetability properties. As a result, the dye will often bead up or more dye is required in order to get an adequately uniform layer of dye in the color isolation well. Attempts have been made to improve the wetability properties of isolation barriers 430 that have been fabricated by patterning photoresist. For example, attempts have been made to reformulate the photoresist and/or plasma treat the photoresist. Such efforts have generally been unsatisfactory. Accordingly, there is a need in the art for improved color isolation wells in color display OLEDs.
In instances where the OLED is a passive matrix color display, both undercut pillars 310 (FIG. 3) and color isolation wells (FIG. 4) are needed. In instances where the display is an active matrix color display, only color isolation wells (FIG. 4) are needed.
Given the above background, it is clear that improved undercut pillars 310 and methods for manufacturing such structures are needed in order to isolate cathode rows in passive matrix OLEDs. Further it is clear that improved isolation barriers 430 and methods for manufacturing such barriers are needed in order to address the problems of wetability and containment in color based OLED displays.